Lithium-ion secondary batteries are in wide use as power sources for small-sized mobile instruments such as mobile phones and laptop computers for having high energy density and excellent charging and discharging cycle characteristics.
In addition, recently, in accordance with consideration of an environmental problem and rising awareness of energy saving, there is also a growing demand for large-sized batteries which are used as power sources in electric vehicles, hybrid electric vehicles, the electricity storage field, and the like and are required to have a large capacity and a long life span.
Generally, a lithium-ion secondary battery is mainly made of a negative electrode in which a carbon material capable of storing and releasing lithium ion is included as a negative electrode active material, a positive electrode in which a lithium composite oxide capable of storing and releasing lithium ion is included as a positive electrode active material, a separator which separates the negative electrode and the positive electrode from each other, and a nonaqueous electrolytic solution which is a nonaqueous solvent having lithium salt dissolved therein.
As exterior materials for lithium-ion secondary batteries, in place of metal cans, thin metal films which can be reduced in weight and thickness and can freely employ a shape compared to metal cans, or laminate films in which a thin metal and a heat-fusible resin film are layered are also in use.
Aiming at increasing the capacity, lengthening the life span, and improving the mass production stability, lithium-ion secondary batteries are required to be further improved in characteristics.
Examples of countermeasures for increasing the capacity of lithium-ion secondary batteries include methods disclosed in Patent Document 1 (Japanese Unexamined Patent Publication No. H6-267539) and Patent Document 2 (Japanese Unexamined Patent Publication No. 2002-319398).
Patent Document 1 discloses a technology of making a positive electrode active material to be a nickel composite oxide by replacing a part of lithium nickelate or nickel with a transition metal.
In addition, Patent Document 2 discloses a technology of mixing a lithium nickel composite oxide in which a heteroelement is introduced and a lithium cobalt composite oxide in which a heteroelement is introduced, for use.